New Membrane Nano-Morphologies for Improved Fuel Cell Operation
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Polymeric membranes play a crucial role during the generation of electricity in hydrogen/air and direct methanol proton-exchange membrane (PEM) fuel cells. The membrane in such devices has three functions: (1) to physically separate the positive and negative electrodes (so there is no electrical short circuit), (2) to prevent mixing of the fuel and oxidant, and (3) to provide a conduit for proton transport between the electrodes. For hydrogen fuel cells, the membrane must exhibit low gas permeability and high proton conductivity. For a direct liquid methanol PEM fuel cell, the ion-exchange membrane must conduct protons and be a good methanol barrier. For any fuel cell, the membrane must have good mechanical properties in the wet and dry states and be chemically stable under fuel cell operating conditions. DuPont’s Nafion® (a perfluorosulfonic acid polymer) has many attractive properties and has been widely studied in PEM fuel cells, but it does not meet all performance criteria. In a hydrogen/air fuel cell, Nafion loses water and the conductivity drops at temperatures 80oC, unless the water activity in the feed gases is near unity. Nafion has also been used in direct methanol fuel cells, but high methanol crossover (permeation) leads to low power output due to cathode depolarization. Polymer nano-morphology manipulation/control is a promising strategy to improve the performance of fuel cell membranes. Two examples of this approach will be discussed: (i) pre-stretched recast Nafion for direct methanol fuel cells and (ii) composite membranes based on proton conducting ionomeric nanofibers. Membrane fabrication methods and the results of membrane characterization tests will be described. Physical property data relevant to PEM fuel cell applications will be related to the membrane’s nanostructure and fuel cell performance data will be presented.